Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate.
Conventionally, the exposed plate is developed with a liquid developer to bare the substrate in the non-hardened or solubilized areas. On-press developable lithographic printing plates have been disclosed in recent years. Such plates can be directly mounted on press after imagewise exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed, allowing the reduction of labor and the elimination of hazardous waste. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.
Traditionally, the plate is exposed with an ultraviolet light from a lamp through a separate photomask film having a predetermined imaging pattern that is placed between the light source and the plate. Laser sources have been increasingly used to imagewise expose a printing plate that is sensitized to a corresponding laser wavelength, allowing the elimination of the photomask film. Suitable lasers include, for example, infrared lasers (such as laser diode of about 830 nm and NdYAG laser of about 1064 nm), visible lasers (such as frequency-doubled NdYAG laser of about 532 nm and violet laser diode of about 405 nm), and ultraviolet laser (such as ultraviolet laser diode of about 370 nm). Laser sensitive plates generally have higher sensitivity (than conventional plates) because of the limited power of current laser imagers.
While laser imagable plates have the advantage of not requiring a photomask film for imaging, such plates often have the drawbacks of limited room light stability, weaker visible images upon exposure, higher sensitivity to the process condition (such as developer temperature or freshness, brush pressure, or lag time between laser imaging and development), limited photospeed for the available laser, or limited durability.
The inventor has found that a simple treatment of the laser imaged plate with a deactivating solution and overall irradiation can be used to cause certain chemical or physical change to the plate without developing the plate, so as to eliminate some of the above drawbacks or to enhance the performance of the plate (such as to improve color contrast, resolution, durability, or imaging consistence).